Electric field light-emitting devices, which have self-luminous property and thus high visibility and excellent display capability and enable high speed response and low-profile, are now attracting the attention as displays for flat displays and the like.
Among others, an organic EL device, using organic compound as phosphor, has characteristic features, as compared with inorganic EL device, that it can be driven at a low voltage, that it can easily produce an enlarged area, and that it can easily produce a desired luminous color by an appropriate choice of coloring matter and, accordingly, the organic EL device is now vigorously developing as a next-generation display.
The EL device using the organic phosphor generates blue luminescence for example, through the application of a voltage of 30 volts to an anthracene evaporated film having thickness of 1 m or less (Thin Solid Films, 94(1982) 171). However, this device fails to produce sufficient luminance even when a high voltage is applied thereto, so that it is required to be further improved to have higher luminous efficiency.
In regard to this, Tang et al. teaches that transparent electrodes (anode), a hole transport layer, an emission region having electron transport capabilities, cathode using metal of low work function are laminated for reduction in voltage and improvement in luminous efficiency, to thereby produce luminance of 1,000 cd/m2 through the application of a voltage of 10V or less (Appl. Phys. Lett. 51 (1987) 913). Used as the phosphor is tris (8-quinolinolato) aluminum complex (hereinafter it is referred to as “Alq”). The Alq is an excellent luminophor having both high luminous efficiency and high electron transport capability.
Further, a device having a three-ply structure wherein an emission region is sandwiched between a hole transport layer and an electron transport layer and a device that obtains luminescence from coloring matter (coumarin derivative or fluorescent dye, such as DCM1, used to the Alq) doped in the emission region is reported (Jpn. J. Appl. Phys., 27 (1988) L269 and J. Appl. Phys., 65(1989) 3610). The report says that it is discovered that an adequate choice of the coloring matter can allow luminous color to change and can produce improved luminous efficiency, as compared with the non-doped one.
In the devices constructed above, all layers are formed by dry process such as a vacuum evaporation method. On the other hand, the method has been proposed of producing the device in the so-called wet deposition process such as a spin coat method and a cast method (Japanese Laid-open (unexamined) Patent Publications No. Hei 3-790 and No. Hei 3-171590).
In this method, at least one material for forming the hole transport layer, the electron transport layer and the emission region is dissolved in appropriate solvent, together with polymer binder. Then, that solution is applied onto the electrode to form the emission region and, thereafter, a further electrode is formed on the emission region in the evaporation method or a like method. Hereinafter, the organic light-emitting device thus produced is called “polymer dispersed light-emitting device”, in contrast to the conventional laminated light-emitting device.
When comparing with the organic light-emitting device produced by the dry process, the polymer dispersed light-emitting device has the following advantages.
(1) Even the material for which it is hard to be deposited in the dry process such as evaporation can be used;
(2) Even the doping of minute amounts for which it is hard to be controlled in the dry process can be performed with ease;
(3) An enlarged area can be produced with ease;
(4) The device can be produced at low costs;
(5) Simultaneous luminescence from luminous molecules is enabled with ease through the introduction of a number of luminous molecules (white luminescence is enabled); and
(6) In the conventional laminated light-emitting device, the layers are each in the amorphous state. In contrast to this, in the polymer dispersed light-emitting device, the materials are dispersed in the polymer binder. Accordingly, the latter is thermally stable as compared with the former.
The conventional polymer dispersed light-emitting devices comprise the emission region in which perinone derivative or tris (8-quinolinolato) aluminum used as the luminous molecules is dispersed ill polyvinyl carbazol or the emission region in which tris (8-quinolinolato) aluminum and tetraphenyl benzidine used as the luminous molecules are dispersed in polycarbonate (Japanese Laid-open (unexamined) Patent Publications No. Hei 3-790, No. Hei 3-171590).
(First Problem)
The polymer dispersed light-emitting device has the advantages mentioned above, while on the other hand, it has the disadvantage that it is low in luminous efficiency, as compared with the conventional laminated light-emitting device.
Specifically, in the laminated light-emitting device, holes are injected from the anode to the hole transport layer, and electrons are injected from the cathode to the emission region of the electron transport capability or the electron transport layer. When these holes and electrons are recombined in the emission region, excitons are formed, and when the excitons are caused to transition to the ground state, the emission takes place. It is to be noted that the electron transport and the hole transport are functionally separated from each other, so that the recombination of the electron and hole takes place only in the vicinity of an interface between the adjoining layers. This can produce the efficient production of the excitons and thus improved luminous efficiency.
Further, through an appropriate choice of the material of the layers adjoining the respective electrodes to minimize injection barriers between the anode and the cathode, the injection of the hole and the electron can be facilitated, and as such can allow the drive at a low voltage.
In contrast to this, the polymer dispersed light-emitting device mainly comprises a monolayer, so that the recombination of the hole and electron and the production of the excitons do not take place locally, differently from the laminated light-emitting device mentioned above. In addition, the barriers for the hole and the electron to be injected from the electrodes are high. These facts make it difficult to improve luminous efficiency.
Thus, the emission site is dispersed over the whole emission region, so that it is hard to balance the injection of the hole and electron and the transport of the same. As a result of this, the probability of the recombination reduces, such that sufficient luminous efficiency is not produced. This indicates that collection of the recombination region of the hole and electron into a specific region leads up to improvement in luminous efficiency.
Though the method of laminating the functionally separated layers is of effective, as mentioned above, in a polymeric system formed by coating, solvent contained in polymer solution of a second layer to be laminated over a first deposited layer must be chosen to prevent the first deposited layer from being dissolved in that solvent.
In addition, as the laminated-layers increase in number, the need to choose the solvent and the need to choose the material soluble in that selected solvent arise increasingly. This arises the problem that the range of choice for the material is further narrowed, thus hindering the effectively improved efficiency.
(Second Problem)
Further, the polymer dispersed light-emitting device involves the problem that when a color panel is produced, it is difficult to do the patterning (distribution of application of color). When the color panel is produced in the dry process such as the vacuum evaporation method, elements of color can be formed in desired positions by an evaporation mask being set on a substrate. In contrast to this, in the wet deposition such as the spin coat method or the cast method, since the emission region is formed over the whole area of the substrate, the above-mentioned patterning cannot be performed.
In regard to this, the patterning using an ink-jet method has been proposed (e.g. Japanese Laid-open (unexamined) Patent Publication No. Hei 10-12377). This proposes that material of the emission region including polymer or the precursor of polymer is discharged from nozzles by the ink-jet method to form a desired pattern.
However, the patterning using the conventional ink-jet method involves the problem that polymer solution or equivalent to be coated is high in viscosity, so that the nozzles of the ink head are easily plugged up and, accordingly, it is difficult to form a micropattern.
To solve the problems mentioned above, the present invention has been made. The present invention provides an organic light-emitting device capable of producing highly improved luminous efficiency and at the same time facilitating the patterning even in the polymer dispersed organic light-emitting device and the producing method thereof.